This invention relates to electro-active devices, and uses therefor. More particularly, the invention concerns novel constructions of electro-active (such as piezoelectric and piezoresistive) devices, some with integral positioning and control mechanisms. The electro-active devices may be used as electromechanical drivers, sensors or generators.
Electro-active devices are those which make use of components that display electro-active propertiesxe2x80x94that is, those in which a component changes shape in response to a change of the appropriate electrical conditions in which the component exists. Equally, of course, the component may produce electrical signals in response to a shape change. The best known, and most developed, of these devices are piezoelectric devices. However, it will be understood that there are a number of other sorts of electro-active device, including those that are electrostrictive (made from a material which contracts on the application of an electric field) or piezoresistive (this latter group being those the electrical resistance of which changes as they change shape). The devices of the invention include those with components that display effects based on such other types of electro-activity.
Early piezoelectric devices, and indeed many in use today, were merely simple blocks of piezoelectric material. If compressed in some direction they produce a voltage across opposite faces in a relevant direction; if, alternatively, a voltage is applied across them then they very slightly change their dimensionsxe2x80x94typically by considerably less than a micron (1xc3x9710xe2x88x926 m).
Devices operating in this manner have found considerable use in various fields. However, there are many occasions when it is desirable for the application of an electric voltage to produce a much greater change in dimensions, of the order of several millimetres, and vice versa. Attempts to achieve this have focussed on a type of device known as a xe2x80x9cbenderxe2x80x9d.
A bender is a construction of piezoelectric device wherein the piezoelectric material is physically in the form of an elongate but relatively thin bar, rather like a ruler, with its associated electrodes along the surface of the bar, and this operating bar is fixedly attached, face to face, onto a substrate in the form of a like bar (which may itself be either of a piezoelectric material or of a non-piezoelectric material). For example, FIGS. 1 and 2 show a known piezoelectric unimorph bender. The bender comprises a flat, uniform layer of active piezoelectric material 1 (shown hatched) bonded face-to-face to a like flat, uniform layer of inactive non-piezoelectric material 2 (shown plain).
When an appropriate electric field is applied across the piezoelectric layer 1 by means of suitably-placed electrodes (not shown, but on either main face of the piezoelectric layer 1), the dimensions of the layer 1 change. In particular, the layer lengthens very slightly. The substrate bar is left undisturbed and so its length is unchanged, or perhaps is made to change in the opposite sense, a bimorph. Expansion of the piezoelectric material, coupled with the restriction placed on it by the unchanged inactive layer 2, causes significant bending of the entire bar in a direction normal to the plane of the bar, as shown in FIG. 2. The movement of one end of the bar relative to the other may be considerable even though the length change is small; it may be many times the length change. For example, using a dual-bar structure 5 cm long, a length change of a fraction of a micron may manifest itself as a tip movement of up to 0.1 mm, or as much as a hundred times the length change. However, the path of the displacement is not linear, because the tip of the device follows a curved path in space.
As already described, on activation a plane bender bends forming a curve which can be described by a radius of curvature and the angle subtended by the ends of the bender. The average length of a bimorph bender does not change, as one part extends while the other part contracts, leaving a neutral axis along the central part of the bender which is the same length as in the inactivated state curved benders are also known, and are typified by that type known as a xe2x80x98Rainbowxe2x80x99. They are shaped such that the thickness of the device is radial, the bender tape being curved about an axis parallel to its width direction. Such a curved bender also bends on activation. The curve becomes tighter, which is equivalent to a smaller radius of curvature, while the subtended angle increases. Further, if the curvature of such a curved bender is circular (that is, the inactivated bender is in the shape of a circle or an arc of a circle), then on activation it bends to give a larger arc of a circle of smaller radius; the angle subtended increases. The radial change is small (microns for radii of curvature of millimetres or centimetres), and independent of bender length. The angle change, however, increases as the bender length increases, and can be quite significant. Thus, if one end of the bender is fixed, the apparent motion of the other end is primarily a rotation. For a circular bender of radius a few centimetres, this rotation may be about one degree or so.
In an extension of this circular geometry, helical benders are also known. In these, the bender is in the shape of a helix, rather like a strip of paper flat-wound around and along a cylinder (a tape-wound helix). As with the circular geometry bender, there is a small radial change, independent of tape length. And also as with the circular geometry case, there is with the helical case a rotational displacement about the axis of the helix, but with a helix the relative displacement of the ends follows a helical, rather than circular, path. There is thus also a small change in the axial length of the helix, dependent on the helix pitch angle. The amount of rotation and hence axial length change increases with bender length, resulting in quite significant rotations and axial displacements in long tape-wound helices. For instance, in a helix with a diameter of about 1 cm, an axial length of several centimetres, and having several helical turns of a bender tape several millimetres wide, the radial change is of the order of microns while the axial length change may be around 1 mm and the rotation may be several degrees.
It would be desirable to provide an electro-active device having a form which allows for large displacement relative to the size and/or weight of the device.
It would further be desirable to provide an electro-active device having a form which provides displacement which is linear in space, or can follow a path which is selectable by design of the device.
According to the present invention, there is provided an electro-active device having an electro-active structure extending along a minor axis which is curved with a total curvature of at least 30xc2x0, the electro-active structure comprising successive electro-active portions extending around said minor axis and arranged with electrodes to bend, when activated, around the minor axis such that bending of the successive portions is concomitant with rotation of the electro-active structure about the minor axis adding incrementally along the minor axis.
Such an electro-active device on activation is displaced out of the plane of the curve. On mechanical activation, the displacement creates an electrical signal on the electrodes, and vice versa on electrical activation. The displacement of the electro-active structure is concomitant with the rotation of the structure and can be understood as follows.
The displacement derives from (a) rotation of the structure around the minor axis and (b) the curve of the minor axis along which the structure extends (hereinafter called the major curve, for ease of reference).
The rotation occurs as follows. Because the successive electro-active portions bend around the minor axis, bending of each portion relatively rotates the adjacent portions around the minor axis. In this way bending of the electro-active portions is converted into rotation of the structure as a whole around the minor axis and vice versa. The rotation adds incrementally along the length of the minor axis. Accordingly there is a net relative rotation between the ends of the structure. When electrically activated an electrical signal applied to the electrodes causes such rotation. In the converse mode of operation when mechanically activated, such rotation generates an electrical signal on the electrodes.
Now consider a small section of the structure along the minor axis. As described above, bending of the portions within the section causes rotation of the structure within the section about the minor axis. The section is curved. As a general point, it will be understood that internal rotation of a curved object creates movement of the object out of the plane of its curve. In the present structure, the rotation within the individual section around the minor axis causes the section to move out of the plane of the curve. This may be visualised as the rotation in the given section displacing adjacent sections because those adjacent sections extend at a slight angle to the given section due to the curve. This is equivalent to an extension or contraction of that section along the direction out of the plane of the curve. It is also equivalent to a change in orientation of the section, that is from an orientation in the plane of the curve to an orientation at an angle to the plane of the curve. In fact, the amount of displacement will be proportional to the degree of rotation within the section and the degree of curvature of the section.
When electrically activated, the net displacement is a summation of the displacements of all the sections of electro-active structure. Vice versa, in the converse mode of operation when mechanically activated, the overall displacement of the structure creates rotation along the minor axis of the structure. In one mode of operation electrical activation generates a rotation of the structure which generates out-of-plane displacement of the structure, or vice versa in a converse mode of operation mechanical activation by out-of-plane displacement generates rotation of the structure which generates an electrical signal on the electrodes.
The displacement is most easily visualised where the minor axis is curved in a regular curve around a geometrical major axis. Such a major curve may be a helix, spiral or an arc of a circle. Rotation of each section causes relative displacement of the ends of that section along the major axis. Therefore, the overall displacement is extension or contraction of the structure parallel to the major axis. However, displacement is achieved by any curve, so the major curve may be of any shape. To achieve a significant effect on activation, the major curve has a total curvature of at least 30xc2x0, most preferably at least 90xc2x0.
Such an electro-active device can provide a large displacement compared to known devices. As a considerable total rotation may be achieved along the length of the minor axis, a correspondingly large displacement may be achieved along the major axis. The amount of extension is proportional to overall length and size of the device. Therefore by appropriate sizing of the device, it is possible to achieve large displacements beyond levels achievable by known benders.
In fact, the displacement of an electro-active structure in accordance with the present invention is quite striking to watch. Millimetres or even centimetres of displacement can be achieved. For example, a structure formed from a 0.5 mm thickness tape wound as a 4 mm diameter minor helix around the minor axis which minor helix itself curves around a major curve which is a segment of about three quarters of a circle of 30 mm diameter has been observed to produce displacement of about xc2x16 mm. Similarly a structure in which the major curve is a 20 turn helix of diameter 30 mm would produce displacement of around xc2x1120 mm. The rotation of the structure around the minor axis is barely visible, but the net effect is a considerable displacement.
The displacement may be controlled by appropriate design of the electro-active structure. For example, a regular structure along the length of the minor axis provides a displacement which is linear in space. This is highly desirable in many applications. In contrast, variation in the structure along the length of the minor axis and/or the shaping of the device to curve around a non-linear major axis allows the path of the displacement to be controlled.
In the general case where either or both the structure along the minor axis or the major curve is not regular, the out-of-plane displacement on activation is generally non-linear. Large displacements in any desired direction or following any desired path can thus be obtained by careful selection of the geometry.
If an electro-active material is used which has a linear field-strain characteristic, then the device will have a linear field-displacement response.
Preferably, the electro-active portions have a bender construction, that is formed from a plurality of electro-active layers, at least one of which is electro-active material. The other layers may be non-active to form a unimorph construction or electro-active to form a bimorph or multimorph construction. The layers will be provided with electrodes arranged for activation of the layers. In general, the layers will be at successive radial positions relative to the minor axis so that bending on activation occurs around the minor axis. Such a bender construction creates a large degree of bending around the minor axis which maximises the net rotation and hence the displacement for a given applied voltage, or vice versa. As the structure is compliant and all the electro-active material can be fully utilised, it produces a large displacement for a given size of device.
A preferable form for the electro-active structure is a continuous electro-active member, such as a tape, which extends along and curves around the minor axis. This form of electro-active structure is particularly easy to manufacture. For example, it may be formed by winding a deformable, continuous electro-active member into the structure.
Preferably, the continuous electro-active member extends as a helix around the minor axis as this is easy to form and maximises efficiency of the device in converting bending to displacement, or vice versa. With a helix it is easy to form a regular structure along the length of the minor axis or to introduce a variation along the length of the minor axis to modify the movement of the device. In general many shapes will provide the necessary rotation around the minor axis, and the turns of the winding may vary in shape, diameter and spacing (in this case the term xe2x80x98axisxe2x80x99 refers to the macroscopic approximate centre line of the winding; the local axis of curvature and radius of curvature vary along the minor axis).
It is also easy to envisage the operation of the device when formed as a helical tape or other continuous member. In this case, bending of the tape can clearly be understood to cause a rotation of the structure as a whole around the minor axis. Furthermore, when the major curve around the major axis is considered, it is easy to visualise the displacement of one end of the helical tape relative to the other end caused by the rotation. For example, where the minor curve extends in a circle or arc of a circle arranged horizontally, if one end is fixed, then on activation in the correct sense, the other end rises vertically, as do all points in between, the amount of vertical displacement rising progressively towards the free end. There is a sideways displacement of the minor axis along which the structure extends manifesting itself as a change in orientation of the minor axis. However, it will be apparent that other structures within the scope of the present invention cause a similar effect.
An alternative form for the electro-active structure is a plurality of discrete electro-active elements connected together. The discrete elements may be separately formed and connected by separate connecting elements. This is particularly advantageous because it allows the mechanical response of the device to be controlled by appropriate selection of the form and material of the connecting element. This means that the desired mechanical response does not restrict the choice of electro-active material.
Alternatively, the elements and connecting portions may be formed from a unitary elongate member.
A device in accordance with the present invention has many uses. It may be used as a driver to convert a signal applied to the electrodes of the electro-active device into relative movement along the major axis. If mounted such that one end is fixed and the other end free, on activation large displacements of the free end result. If a mechanical load is applied to one end, activation causes a force to be developed acting against that mechanical load, thus forming a linear actuator. If, following electrical activation with a non-zero drive voltage, the terminals are open-circuited, the device will maintain its output force (if the mechanical load is static) for a significant time determined by the internal leakage current of the piezoelectric structure. The device acts mechanically like an elastic coil-spring if not electrically activated.
In the converse mode of operation of the electro-active material, the device may be used as a sensor to convert relative movement along the major axis into a signal on the electrodes of the electro-active device. If the terminals are connected to a high-impedance electrical detector or measurement circuit, and the device is subjected to an external axial force, then it will produce a measurable output voltage proportional to the axial compression or expansion of the device caused by that force. Such devices may be used as a force sensor, or a displacement sensor.
Similarly, the device may be used as a generator to convert relative movement into a voltage on the electrodes of the electro-active device.
The major and minor axes are of course imaginary axes but are useful for visualising and defining the device. In regular geometries the axes may be the geometrical axes of curvature or symmetry, but in general they are any axes about which the structure extends.